Many mobile devices, and in particular, mobile telephones, incorporate cameras based on silicon image sensor technology. Besides functioning as a camera, these mobile devices often provide functions such as, for example, a mobile telephone, a Bluetooth communication device, or an MP3 (Moving Pictures Experts Group 3 or MPEG3) player. Because these mobile devices need to increase the time that they can operate on a single battery charge, any function that is not required is usually switched off or put into a low power mode. Consequently, any reference voltages or current supplies on the device may vary slightly depending on the functions that are currently operating on the device and/or on the state of the battery itself. Other causes of power fluctuations can include electromagnetic interference from other devices and/or from other functions within the same device.
An image sensor, or pixel, requires reference voltage and/or current supplies to enable the switching of transistor gates, and the readout of a photodiode or other light-detecting device. An amplifier acting as a regulator of the battery supply is normally used to supply the reference voltages. Variance in the reference voltages can affect the value read out from the pixel. Typically, the battery supply voltage can vary by ±100 mV whereas the sensitivity associated with a pixel is typically ±100 μV. Hence, the reference voltages may vary more than the sensitivity of the pixel.
To enable an image to be created, the values of an array of pixels are read out. This is usually done one row at a time. If any of the reference supplies vary during the read out of the array of pixels, inconsistencies between individual pixels can occur. In general, the image quality can be adversely affected by changes in the reference voltage and current supplies.
For example, consider a three-transistor (3T) pixel, as shown in FIGS. 1a and 1b, operating with a double sampling timing. To describe the intended operation of the pixel, it will be assumed that all reference supplies are ideal.
For a first sample reading, the pixel is first reset, by turning on transistors MRD and MRST. This causes a node VPIX to be pulled up to a pixel reference voltage VRT. The pixel is then released, by turning off transistors MRD and MRST, so that it can start integrating. By doing so, a charge injection from MRST causes a drop in the voltage of VPIX, thus:VPIX=VRT−Vinj 
The pixel's capacitor CPIX is then discharged by the photo-diode current Ipd, leading to:
  VPIX  =      VRT    -          V      inj        -          (                                    I            pd                    ·          t                CPIX            )      
Once the integration time is over, MRD is turned on and the buffered value of VPIX, that is an output node VX, is read out. Ideally it is equal to:
  VX  =            VPIX      -                        V          GS                ⁡                  (          MBUF          )                      =          VRT      -              V        inj            -              (                                            I              pd                        ·            t                    CPIX                )            -                        V          GS                ⁡                  (          MBUF          )                    
The first sample measured value of VX contains not only information about ambient light, but also information about the pixel's offset and is stored as VSIG.
To eliminate the offset, a second sample reading is made by turning on MRST (VPIX=VRT), then turning it off (VPIX=VRT−Vinj). This leads to a second sample measurement, referred to as VBLK, and is equal to:VBLK=VX=VRT−Vinj−VGS(MBUF)
Information about luminosity can be determined by subtracting VSIG from VBLK. This leads to a value of
VLUM:
  VLUM  =            VSIG      -      VBLK        =          (                                    I            pd                    ·          t                CPIX            )      
Variations on how RSTHI, RDHI, VRT, and ICOL affect the measurement of light will now be examined. At first the pixel is reset:VPIX=VRT+∂VRT1 
The pixel is then released for integration, which leads to a sudden decrease in VPIX. However, this injection is not ideal and has a dependency on the supply RSTHI such that:VPIX=VRT+∂VRT1−Vinj−(A·∂RSTHI1·Vinj)
The variable A represents a technology dependent constant related to the supply RSTHI.
The pixel then integrates, leading to:
  VPIX  =      VRT    +          ∂              VRT        1              -          V      inj        -          (              A        ·                  ∂          RSTHI                ·                  V          inj                    )        -          (                                    I            pd                    ·          t                CPIX            )      
Once the integration time is over, VX is read out and is equal to VPIX−VGS (MBUF). However, the transistor MRD is not a perfect switch, and an offset dependant on RDHI will be introduced. This offset will be defined by B×δRDHI1, where B represents a technology dependent constant related to the supply RDHI.
The VGS drop of MBUF is dependant on ICOL, thus an offset δVGS (MBUF)1 needs to be added to VGS (MBUF). This first sample measurement VSIG is now:
  VSIG  =      VRT    +          ∂              VRT        1              -          V      inj        -          (              A        ·                  ∂                      RSTHI            1                          ·                  V          inj                    )        -          (                                    I            pd                    ·          t                CPIX            )        -                  V        GS            ⁡              (        MBUF        )              -          ∂                                    V            GS                    ⁡                      (            MBUF            )                          1              +          (              B        ·                  ∂                      RDHI            1                              )      
To eliminate the offsets of the pixel, a second measurement is made by turning on MRST (VPIX=VRT+δVRT2), then turning it off:VPIX=VRT+∂VRT2−Vinj−(A·∂RSTHI2·Vinj)
This value is then measured as VBLK, which is equal to:VBLK=VPIX=VRT+∂VRT2−Vinj−(A·∂RSTHI2·Vinj)−VGS(MBUF)−VGS(MBUF)2+(B·∂RDHI2)
As done previously, the information about luminosity can be extracted by subtracting VSIG from VBLK. This leads to the following value of VLUM:
  VLUM  =            (                                    I            pd                    ·          t                CPIX            )        +          ∂              VRT        2              -          ∂              VRT        1              -          A      ·                        V          inj                ⁡                  (                                    ∂                              RSTHI                2                                      -                          ∂                              RSTHI                1                                              )                      +          B      ⁡              (                              ∂                          RDHI              2                                -                      ∂                          RDHI              1                                      )              -          (                        ∂                                                    V                GS                            ⁡                              (                MBUF                )                                      2                          -                  ∂                                                    V                GS                            ⁡                              (                MBUF                )                                      1                              )      Clearly, VLUM is affected by supply variations and the quality of the measurement is affected.
Previously, little or no attention has been paid to the degradation caused by varying reference supplies to an image sensor. Research has instead concentrated on other aspects of degradation, such as fixed pattern noise or reset noise. However, as mobile devices are integrated with many new circuits and functions, this source of image quality loss is likely to become more prevalent.